The present invention relates to a method of efficiently decreasing the amount of unreacted monomer remaining in a polymer without requiring a special apparatus or a complex process, and a toner resin having for its constituent component a polymer in which the amount of unreacted monomer has been decreased by that method.
This application is based on a patent application filed in the country of Japan (Japanese Patent Application No. Hei 11-371950), and the described contents of said Japanese patent application are incorporated as a part of the present specification.
Vinyl polymers are typically produced by polymerization of vinyl monomer by a polymerization method such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization or dispersion polymerization and so forth. In the case of these polymerization methods, it is difficult to completely polymerize the vinyl monomer at a rate of polymerization of 100%. Thus, in many cases, unreacted monomer that has not undergone polymerization usually remains in the polymer. In addition to impairing the mechanical characteristics, thermal characteristics, chemical characteristics and so forth of the polymer, unreacted monomer remaining in the polymer also causes problems which the generation of an unpleasant odor during handling of said polymer and when the polymer is made into a final product. Consequently, several methods have been proposed in the prior art for removing unreacted monomer from polymers.
For example, it is pointed out in Japanese Patent Application, Second Publication No. Hei 4-55203 that, when unreacted monomer remains in a polymer, the problem results in which a foul odor is generated during melting and kneading processing and when preparing a polymer solution. In order to solve this problem, a method for removing unreacted monomer is proposed in which an aqueous slurry containing polymer particles is distilled in the presence of suspension dispersant and non-ionic surfactant or polypropylene glycol or its derivative.
In addition, a method for decreasing unreacted monomer is proposed in Japanese Patent Application, First Publication No. Hei 1-70765 that is used as a method for removing unreacted monomer from a toner resin used in an electrophotographic developer in which, following suspension polymerization, the resulting suspension is heated at a temperature equal to or higher than the glass transition temperature of the resulting resin and water in an amount of 5-50 wt % based on the amount of water present at completion of polymerization is distilled from the suspension in the form of water vapor.
Moreover, as a method for removing unreacted monomer and other volatile components from a modified styrene polymer mixture, Japanese Patent Application, First Publication No. Sho 61-228012 proposes a method for removing unreacted monomer and volatile components in two stages by two stages of a volatile component separation apparatus having a constitution in which a multipipe heat exchanger type preheater and volatilizer are coupled directly.
The methods described in the above Japanese Patent Application, Second Publication No. Hei 4-55203 and Japanese Patent Application, First Publication No. Hei 1-70765 both attempt to remove unreacted monomer by steam distillation. Thus, these methods have the problem of requiring a large amount of thermal energy for steam distillation, thereby increasing the cost of removing unreacted monomer.
In addition, the method described in Japanese Patent Application, First Publication No. sho 61-228012 requires a special apparatus and other incidental apparatuses such as a vacuum pump for degassing, while also requiring energy consumption for preheating the polymerization reaction mixture. Consequently, this method has the problem of increasing the processing cost in the same manner as described above.
Thus, the object of the present invention is to provide a method for inexpensively and efficiently decreasing the amount of unreacted monomer in a vinyl polymer without requiring a special apparatus or thermal energy and so forth.
The method for decreasing unreacted monomer in vinyl polymer of the present invention comprises: adding to an aqueous dispersion in which a vinyl polymer is dispersed in an aqueous medium in the form of particles a water-soluble inorganic salt that is able to solubilize unreacted monomer contained in said vinyl polymer followed by solubilizing unreacted monomer and removing unreacted monomer together with aqueous medium. According to this method, the amount of unreacted monomer in the polymer can be inexpensively, efficiently and reliably decreased without requiring a special apparatus or thermal energy and so forth.
In addition, in the method of decreasing unreacted monomer in vinyl polymer of the present invention, the above water-soluble inorganic salt is preferably used within a range of 0.5-30 parts by weight relative to 100 parts by weight of vinyl polymer in the aqueous dispersion. In addition, the water-soluble inorganic salt is preferably anhydrous sodium bisulfite. Moreover, solubilization of unreacted monomer by water-soluble inorganic salt is preferably carried out at a temperature within the range of the glass transition point of the polymer contained in the aqueous dispersion to 150xc2x0 C.
In addition, the toner resin of the present invention has for its constituent component a vinyl polymer in which the amount of unreacted monomer has been decreased by the above method. In the case of being used as a toner resin in an electrophotographic method, vinyl polymer obtained according to the method of the present invention solves the problem of the generation of a foul odor during toner production and during fixation when copying, while also have other superior characteristics such as contributing to improvement of toner characteristics.